194 PAET II. THE INTIMATE STRUCTURE OF PLANTS. [ 35- 



the primary, and in this way a compact ring of wood and of hast is formed. 

 These secondary bundles are of course destitute of protoxylem and proto- 

 phloem. 



In roots the secondary vascular tissue is developed in essentially the same 

 manner as in the stem ; the wood inwards, the bast outwards, from the cam- 

 bium-layer ; and the same forms of tissue are produced. It is, however, only in 

 certain cases (e.g. Taraxacum, Scorzonera hispanica, Kubia, Taxus, Cupressus, 

 etc.) that the cambium of the root produces wood internally, and bast extern- 

 ally, over its whole surface, so that a complete ring of secondary vascular 

 tissue is formed : in most cases (e.g. Centranthus, Tropseolum, Urtica, Cucur- 



bita, Phaseolus, Convolvulus, Clusia, etc.) 

 secondary vascular tissue is formed only 

 opposite to the primary bast-bundles, 

 whereas, opposite to the primary wood- 

 bundles, the cambium produces only 

 ground-tissue, thus giving rise to broad 

 medullary rays opposite to these bundles 

 (Fig. 146). 



The Tissues developed from the 

 Cambium. In stems and roots in 

 which the growth in thickuess is 

 normal, the cambium gives rise to 

 secondary wood, secondary bast, and 

 secondary conjunctive tissue (me- 

 dullary rays). 



Tlie structure of the secondary wood 

 differs essentially from that of the 

 primary wood only in that it in- 

 cludes no spiral or annular vessels 

 resembling those of the protoxylem 

 (see p. 176). It always includes 

 tracheal tissue ; nearly always wood- 

 parenchyma (see p. 132) ; frequently 

 sclerenchyma : the cell-walls of all 

 these forms of tissue are usually 

 more or less completely lignified. 



The secondary tracheal tissue may 

 consist either solely of tracheae (e.g. 

 Platanus, Fraxinus . excelsior and 

 Ornus, Citrus, Viscum, Hydrangea); 

 or solely of tracheids (e.g. Coniferae, Drimys Winteri) ; or, as is 

 generally the case, of both tracheae and tracheids. The cell- walls 

 of the tracheal tissue are, as a rule, marked with bordered pits ; 



FIG. 146. A Transverse section of 

 a young root of Phaseolus multiflorus : 

 pr cortex ; m pith ; x stele ; g primary 

 xylem-bundles ; b primary phloem- 

 buudles. B Transverse section of an 

 older root of the same plant, which 

 is growing in thickness : b' secondary 

 bast ; fe periderm : the four rays ex- 

 tending to near the centre consist 

 of secondary ground-tissue, and cor- 

 respond in position to the primary 

 wood-bundles. (Slightly magnified; 

 after Sachs.) 



